The present invention relates to superabsorbent thermoplastic polymer blends and to meltblown or spunbonded webs prepared therefrom.
Disposable absorbent products such as diapers, sanitary napkins, tampons, incontinence products, and the like typically are comprised of a batt or absorbent portion which is wrapped with a liner. The batt typically is comprised primarily of cellulose fibers and the liner usually is a nonwoven web of a polyolefin such as polyethylene or polypropylene.
In the past, significant efforts have been made to find ways to make such disposable absorbent products more efficient or more appealing to the consumer. Much of such efforts have focused on increasing the absorbent capacity of the product on a unit mass basis while at the same time increasing the ability of the product to retain absorbed fluid. The ability of a product to remove and keep body fluids from the skin is perceived as a desirable attribute and is believed to be a factor in the reduction of such skin problems as diaper rash.
Such increased absorbent capacity and fluid retention typically have been accomplished by incorporating a superabsorbent material into the absorbent batt. The superabsorbent material usually is in particulate form. Unfortunately, particulate superabsorbents often migrate in or fall out of the absorbent product and/or exhibit gel-blocking, a phenomenon which prevents the migration of fluid into the central portion of the superabsorbent particle. Moreover, the use of particulate suberabsorbents complicates the manufacturing process and the particulate nature of the superabsorbents limits the applications for the superabsorbents.
Superabsorbent materials, also referred to as hydrogels, frequently are based on acrylate and methacrylate polymers and copolymers. Other hydrogels are based on starch or a modified starch. Hydrogels prepared from hydrolyzed crosslinked polyacrylamides and crosslinked sulfonated polystyrenes or based on maleic anhydride (or similar compounds) have been described. Finally, still other hydrogels are based on polyoxyalkylene glycols and include polyurethane hydrogels.
The known polyoxyalkylene glycol-based superabsorbents generally tend to come within one of the following classes, at least in so far as they are reasonably related to the present invention:
(1) crosslinked poly(oxyalkylene) glycols;
(2) crosslinked isocyanate-capped poly(oxyalkylene) glycols, i.e., polyurethanes or polyureas;
(3) polyurethanes prepared from polyfunctional prepolymers or resins and diisocyanates;
(4) polyurethanes prepared from isocyanate-capped polyesters or poly(oxyalkylene) glycols and difunctional extenders;
(5) polyurethanes prepared from poly(oxyalkylene) glycols, isocyanate-capped low molecular weight prepolymers, and polyfunctional low molecular weight components.
Class 1--Crosslinked Poly(oxyalkylene) Glycols
U.S. Pat. No. 3,783,872 describes absorbent pads containing insoluble hydrogels. The hydrogels can be present in the pads as a powder or as a film. The hydrogels are comprised of crosslinked poly(alkylene oxide). Suitable materials are stated to include, by way of illustration, a poly(alkylene oxide), such as poly(ethylene oxide); poly(vinyl alcohol); poly(vinyl methyl ether); copolymers of maleic anhydride and ethylene; and copolymers of maleic anhydride and vinyl methyl ether. The polymers are crosslinked by ionizing radiation.
An apparently preferred group of polymers includes poly(ethylene oxide), copolymers of ethylene oxide and propylene oxide, and alkyl-substituted phenyl ethers of ethylene oxide polymers in which the alkyl groups may be methyl and/or butyl.
The polymers which can be used also are described in terms of reduced viscosity, rather than by molecular weight. The polymers apparently can have average molecular weights of less than about 150,000 to more than about 10,000,000.
Class 2--Crosslinked Isocyanate-Capped Poly(oxyalkylene) Glycols, i.e., Polyurethanes or Polyureas
U.S. Pat. No. 3,939,105 describes microporous polyurethane hydrogels. The hydrogels are the reaction products of a poly(oxyalkylene) polyol having an average molecular weight of up to about 25,000 and organic diisocyanate which have been lightly crosslinked with water or an organic amine. In practice, the polyol is reacted with the diisocyanate to give an isocyanate-capped polyol. Before the isocyanate-capped polyol is crosslinked, a liquid nonsolvent is added thereto in an amount which will not result in precipitation. It is the addition of the nonsolvent which results in the production of the microporous hydrogel. The nonsolvent typically is an aliphatic hydrocarbon or a dialkyl ether.
The disclosure of U.S. Pat. No. 3,939,123 is similar to that of the foregoing patent, except that a nonsolvent is not employed.
In a variation of the procedures disclosed in the two preceding patents, U.S. Pat. No. 3,940,542 describes the extrusion of a solution of the isocyanate-capped poly(oxyalkylene) polyol of such preceding patents into a coagulant or crosslinking bath containing water or an organic polyamine as a crosslinking agent to produce water swellable, lightly crosslinked hydrogel polymer tapes or fibers. U.S. Pat. No. 4,209,605 describes another variation in which hydrogels are produced by charging preselected feeds containing the poly(alkyleneoxy) polyol, diisocyanate, and catalyst to a reaction zone, extruding the resulting high viscosity polymer through a suitable die, and allowing crosslinking to take place by exposure to atmospheric humidity.
U.S. Pat. No. 4,182,827 describes a method of increasing the wetting rates of the hydrogels disclosed in the above three patents. The wetting rates are enhanced by treating the surface of the solid hydrogel with certain ketones, alcohols, organic amines, aromatic hydrocarbons, or aqueous alkali metal hydroxide solutions.
Class 3--Polyurethanes Prepared from Polyfunctional Prepolymers or Resins and Diisocyanates
Hydrophilic polyurethane polymers are described in U.S. Pat. No. 3,822,238. They are prepared by reacting a diisocyanate with a polyfunctional prepolymer or resin. The prepolymer or resin can be, among other things, an adduct of ethylene oxide, propylene oxide, ethylene imine, propylene amine, dioxolane, or a mixture thereof with a polyhydroxy compound; a hydroxy carboxylic acid; a low molecular weight, hydrolyzed poly(vinyl acetate), polyacrylic acid, or polymethacrylic acid; or mixtures thereof. Examples of polyhydroxy compounds include ethylene glycol, propylene glycol, glycerol, trimethylolpropane, erythritol, pentaerythritol, anhydroenneahepitol, sorbitol, mannitol, sucrose, and lactose. See also U.S. Pat. No. 3,975,350 which describes a carrier system employing the hydrophilic polyurethane polymers of the first patent.
Class 4--Polyurethanes Prepared from Isocyanate-Capped Polyesters or Poly(oxyalkylene) Glycols and Difunctional Extenders
Segmented urethane polymers are described in U.K. patent ppplication GB No. 2,154,886A. Briefly, a polyester or polyether glycol having a molecular weight of at least about 200 is reacted with an excess of organic diisocyanate to form an isocyanate-terminated prepolymer. The prepolymer then is reacted with a difunctional extender and, optionally, with a very small proportion of a monofunctional material which acts as a molecular weight regulator.
A polyether glycol apparently is preferred, such as a poly(tetramethylene ether) glycol having a molecular weight above about 600, e.g., from about 800 to about 5000.
The diisocyanate usually is an aromatic diisocyanate such as 4,4'-diphenylmethane diisocyanate and toluene diisocyanate. An excess of diisocyanate is employed, e.g., from about 1.2 to about 1.9 moles of diisocyanate per mole of glycol.
The difunctional extender has two groups which are reactive with isocyanates. The extender can be a diol, an amine having at least one amino hydrogen per amino group, or water. Examples of suitable extenders include water, 1,4-butanediol, diethylene glycol, ethylene glycol, ethylenediamine, diphenylmethane diamine, 1,3-cyclohexylene diamine, 1,4-cyclohexylene diamine, and the like.
Similar polymers are described in U.S. Pat. Nos. 2,929,800, 2,929,804, 3,428,711, and 3,557,044.
Class 5--Polyurethanes Prepared from Poly(oxyalkylene) Glycols, Isocyanate-Capped Low Molecular Weight Prepolymers, and Polyfunctional Low Molecular Weight Components
A polyurethane similar to the polyurethane hydrogels described above is disclosed in U.K. patent application GB No. 2,157,703A. The material is described as useful for coating fabrics and clothing, with no mention of water-absorbing properties. According to the reference, the polyurethane is formed from a reaction mixture comprising an isocyanate-terminated prepolymer, a polyol component containing at least 25 percent by weight of polyoxyethylene units based on the total weight of constituents, and a low molecular weight constituent having an active hydrogen functionality of at least two. If desired, the viscosity of the reaction mixture can be increased by adding one or more additional low molecular weight constituents having a functionality of at least two and preferably three; in the preferred case, it is clear that the additional constituent is functioning as a crosslinking agent.
The prepolymer is formed from the reaction product of a polyisocyanate containing at least two isocyanate groups per molecule with a low molecular weight component having an active hydrogen functionality of at least two. Such component can be a diamine, dihydrazide, diamide, diol, dithiol, dicarboxylic acid, disulfonic acid, or a mixture thereof. Diols are preferred, with representatives examples including thiodiglycol, ethylene glycol, diethylene glycol, and 1,4-butanediol. Trifunctional compounds can be included, such as trimethylolpropane, diethylenetriamine, and compounds having two or more different types of functional groups. Preferably, such low molecular weight component has a molecular weight of not more than 200.
The polyisocyanate used to prepare the prepolymer can be any of those known to be useful for preparing polyurethanes. Examples include toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, mixtures of the foregoing two compounds, 1,6-hexamethylenediisocyanate, 1,5-naphthalenediisocyanate, 4,4-diphenylmethanediisocyanate, 1,4-cyclohexanediisocyanate, 1,4-phenylenediisocyanate, m- and p-tetramethylxylyldiisocyanates and mixtures thereof, isophoronediisocyanate, and 4,4'dicyclohexylmethanediisocyanate. The last two compounds are preferred.
The polyol component contains at least 25 percent polyoxyethylene units. The preferred polyoxyethylene-containing compound is a polyethylene glycol having a molecular weight of from about 400 to about 2000. Other suitable compounds include block copolymers of ethylene oxide with other 1,2-alkylene oxides, such as propylene oxide and butylene oxide; and copolymers formed by reaction of ethylene oxide with polyols, polyamines, and polythiols.
The polyol component may consist in part of substances which do not contain polyoxyethylene units, such as polyester polyols and polyether polyols. Examples of the former include polycaprolactone diols and polyesters prepared from a dicarboxylic acid such as oxalic, maleic, succinic, adipic, suberic, sebacic, and the isomeric phthalic acids and a polyol such as ethylene glycol, diethylene glycol, 1,4-butanediol, 1,6-hexanediol, mixtures thereof, glycerol, trimethylolpropane, pentaerythritol, sorbitol, and sucrose. An example of the latter, which is preferred, is polytetramethylene glycol.
The low molecular weight constituent in general can be the same type of compound as the low molecular weight component already described.
If desired, crosslinking agents such as triisocyanates and melamine-formaldehyde resins also can be employed.